Function concat 

pub fn concat(
    segments: &[PathBuf],
    opts: &ExportOptions,
) -> Result<(), ExportError>

Joins video files encoded with the same settings (segments, in order) into opts.out without re-encoding them, and adds opts.audio. Used by parallel rendering: each segment is encoded on its own, and the result does not depend on how many ran at once.

Errors

If ffmpeg is missing or fails (its log is included).